Microscopic

Ooids Aragonite Layers Dissolving - Freshwater Calcite Cement - Pleistocene (O/14/Ple/Bah)

10x
2.5x
2.5x
5x
5x
5x
2.5 wave plate
5x wave plate
Outstanding feature Meniscus cements & concentric aragonite ooids with surface coats dissolving. Abundant bioclasts.
Folk Classification Bio oosparite
Dunham Classification Bioclastic oograinstone
Common Grains Bioclasts, ooids, botryoidal grapestones
Sorting Well sorted
Rounding Well rounded
Micritization & envelopes Micritized nucleae, often earlier ooids & botryoidal grains
Cement & diagenetic fabric Meniscus low magnesian calcite rims & locally equant spar fill.
Porosity 30% Interparticle & intraparticle porosity
Depositional Setting Subaerial exposure of an open marine high energy beach sequence
Diagenetic history Cemented by marine rim with vadose freshwater calcite filling over this, while aragonitecortex of ooids dissolve

The intial cement appears to be isopachus marine rims. This underwent diagenesis and was superimposed on by meniscus spar cements related to evaporation of freshwater charged with dissolved aragonite within these marine sediments. These grains formed on an exposed carbonate shelf and were transported by waves to their current postion in a now uplifted Pleistocene beach. The aragonite ooids have concentric layering of cortex about bioclasts and micritized peloid nulceae (some after ooids). Botryoidal grains are transported grapestones that form nucleae and are enclosed by aragonite ooid coating. Darkened spots within ooid layers are cyanobacteria colonies that cause local micritization.


Monday, August 27, 2018
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